Low wattage Tremolo TMB reverb

[Bieworm]

That VoCo on the screen voltage looks interesting,have’nt seen any feedback though from anyone who has used it.

I guess we'll find out

[TriodeLuvr]

Jack, is the ECL80 a good way to go? Is the 125A a useable OT for that application? Any opinions? Jukeboxrevival sells these dirt cheap...
https://www.jukebox-revival.eu/catalogs ... t/?q=Ecl80

The ECL80 can't be used for a push-pull TMB-style amp from what I see in the data sheet. The pentode and triode have a shared cathode. There's no way to separate them to create a long tailed pair PI driving a push pull output. Actually, I suspect the shared cathode configuration is the reason they're inexpensive. You can build a push pull amp with these tubes, but it requires trickery and is unlikely to produce the tone you want.

The Hammond 125 series has a very limited low end response with some impedance combinations. The 125A looks OK if the 8 ohm output is used in conjunction with the 11.6K (or less) primary taps.

For the power level you're asking about, triodes are probably more useful. A tube like the 6BL7 (dual triode, octal base) or a pair of 12B4 (single triode, 9-pin miniature) would be excellent. Both types are characterized by low Rp, so they don't require the use of high-Z transformer taps.



Jack

[TriodeLuvr]

Just an afterthought, one more consideration regarding the 125A is its limit of 20 mA per side. I would move up to the 125C, which is rated 45 mA. Here in the States, there's very little difference in the price of any transformer in this series.

Jack

Note: The Hammond site is inconsistent regarding the maximum standing current of these two transformers. One page specifies them as I listed above, another states 25 mA and 60 mA respectively for the 125A and 125C.

[Bieworm]

thanks Jack... but I'm not going that route anymore. I've convinced the man the best way to go is the 18 watt. I'm going to do some testing with the different secondary taps to see if that is a better alternative for attenuating things. The only concern is the tremolo operation at lower voltages.. but since tremolo isn't that important to him it's no disaster if it don't wiggle at lower B+ settings.
OTOH I could look into the mosfet IRF840 system that the toneking has. That tremolo is pretty darn good too...
Lots to think about for now.. even been thinking about building him a superlite TMB with reverb for that matter

[JMPGuitars]

thanks Jack... but I'm not going that route anymore. I've convinced the man the best way to go is the 18 watt. I'm going to do some testing with the different secondary taps to see if that is a better alternative for attenuating things. The only concern is the tremolo operation at lower voltages.. but since tremolo isn't that important to him it's no disaster if it don't wiggle at lower B+ settings.
OTOH I could look into the mosfet IRF840 system that the toneking has. That tremolo is pretty darn good too...
Lots to think about for now.. even been thinking about building him a superlite TMB with reverb for that matter

Definitely go with the SLTMB if he doesn't care about tremolo. It's a great amp, and then you won't have your trem voltage concerns.

[TriodeLuvr]

thanks Jack... but I'm not going that route anymore. I've convinced the man the best way to go is the 18 watt. I'm going to do some testing with the different secondary taps to see if that is a better alternative for attenuating things. The only concern is the tremolo operation at lower voltages.. but since tremolo isn't that important to him it's no disaster if it don't wiggle at lower B+ settings.
OTOH I could look into the mosfet IRF840 system that the toneking has. That tremolo is pretty darn good too...
Lots to think about for now.. even been thinking about building him a superlite TMB with reverb for that matter

I'll be interested to see how this all works out. I'm interested in building a lower power amp when my current hi-fi projects are done, and the list of possible features is pretty long. I have a lot of tubes on hand, so almost any combination of pentodes and triodes in nearly any configuration is possible. I'm leaning toward push-pull triodes after hearing your 1W amp, but that's just one of the many decisions that have to be made. It probably won't be a TMB-style, because I'm not very enthusiastic regarding the limited tonal variation available with this type of tone stack. It's a must for re-creating the sound of the early Marshalls, but otherwise not necessarily the best for my purpose, especially now that I have the 36W.

Please post your progress on this project so I can steal as many of your ideas as possible.

Jack

[Bieworm]

Ok.. problem!!
Just wanted to order those transformers...turns out Ampmaker is on holiday until september...
But thinking on... theoretically I could use half of the winding of a 300-0-300V secondary and rectify that to get 212V instead of 350V, right? A full wave rectifier of course. Is it then absurd to make that switchable from full winding to half, given I switch off the amp to do that?
BTW, it's gonna be the superlite TMB reverb

[TriodeLuvr]

The red notation is mine...




Jack

[geoff 1965]

That’s a cool idea Jack!
Personally I would opt for Vvr on the power & phase section,the customer will have more scope i.e. he can dial in full power for headroom or lower power for breakup.at half power you are stuck with the tone/breakup of that voltage.
decisions decisions Bieworm!

[Bieworm]

That’s a cool idea Jack!
Personally I would opt for Vvr on the power & phase section,the customer will have more scope i.e. he can dial in full power for headroom or lower power for breakup.at half power you are stuck with the tone/breakup of that voltage.
decisions decisions Bieworm!

You're right Geoff. You've done that before. Have any good schematic and tips of doos n don'ts ?

[geoff 1965]

The schematic is in the downloads section Bieworm,the spec for the mosfet is a NTE2377 which is n-channel,900V,8 amp,150W dissipation,TO-3P type.they are hard to find and expensive but the Toshiba 2SK2968 is very close in spec,there are others as well if you search.
Needs to be bolted to the chassis with silicon heat transfer pad insulating between,you can get that in 100 x 100 x 2 mm sheets on eBay.
You can adjust the minimum voltage with the resistor to ground from the 1M pot,100K gives you roughly 10% of max i.e. 350V max voltage will be around 35V.i upped it to 220K on my amp so minimum voltage was around 60V but you can experiment with that yourself.
I did draw out a layout somewhere if I find it I will post ithttps://www.18watt.com/files/18_watt_liteiib_vvr_pa_pi_141.gif
Also if you use a dual axial electrolytic after the can cap you add the filtering to the preamp that is cut out i.e.the first half of the dual can filters the vvr & preamp,the second half filters post vvr to the power & phase section.

[TriodeLuvr]

Well, I only posted that as a solution to Bieworm's question about using half the winding. This technique provides half voltage, but it doesn't cut the available power in half like using half the winding. It's a better option than using half the transformer, regardless of the purpose for doing that. You might want to keep it in mind in case you ever need half-voltage for a preamp/driver stage, but don't want to waste so much power and heat across dropping resistors.

Jack

[geoff 1965]

That method is definitely going in the build notes Jack,I actually had the transformer Bieworm was planning to use and to be honest the half power option just gave a little earlier breakup,the tone was the same as full and it was just as loud!
the vvr gives you more scope and lower volume to play with.

[Bieworm]

And this one? https://www.tube-town.net/ttstore/kit-t ... lator.html

[geoff 1965]

The dissipation and drain are a lot lower on that mosfet Bieworm @ 75W & 4.5 amp,suitable for just the screen voltage but are you going to be the guinea pig that tries it out?